1. Field of the Invention
The present invention relates to a diesel particulate filter apparatus for collecting particulates from exhaust gas discharged from a diesel engine with the filter thereof.
2. Description of the Prior Art
A diesel particulate filter made of, e.g., cordierite (2MgO.multidot.2Al.sub.2 O.sub.3 .multidot.5SiO.sub.2) as the material thereof is known as a conventional exhaust gas treatment filter for collecting particulates, i.e., particulate substances, such as carbon, soot and hydrocarbons (HC) from exhaust gas discharged from a diesel engine.
A filter made using as the material thereof an inorganic fiber instead of cordierite is also known. In a diesel particulate filter apparatus having a filter made of an inorganic fiber, the filter is lightweight and is produced by laminating an inorganic fiber material and locally weaving an inorganic fiber material into the resulting laminate material to effect felting thereof. The filter made of the inorganic fiber material can advantageously be miniaturized as compared with a honeycomb filter made of cordierite because the former enables particulates such as black smoke in exhaust gas discharged from a diesel engine to be trapped in the inner part of the filter as well as on the surface thereof.
However, the conventional diesel particulate filter apparatus having the filter produced using the inorganic fiber, though advantageous in the foregoing respect, is not only fragile when subjected to vibrations and impacts due to the brittleness of the inorganic fiber material itself constituting the filter so as to create a fear of the filter being broken, but such a filter is also problematically high in cost.
Particularly in the step of felting the inorganic fiber and in the step of assembling a diesel particulate filter apparatus of an inorganic fiber type, the high modulus of elasticity of the inorganic fiber makes it brake and become separated into fine pieces during the felting step and the assemblying step, and the fine pieces of the separated fiber are scattered and deteriorate the working environment to cause problems such as irritation of the skins and throats of workers.
On the other hand, the conventional diesel particulate filter made of cordierite collects black smoke on the surface of the filter, so that the pressure loss of exhaust gas is rapidly increased in keeping with the progressive collection of particulates. Accordingly, the area of the filter necessary for collection of particulates is inevitably increased to disadvantageously enlarge a diesel particulate filter apparatus itself. Meanwhile, the filter used for collection of particulates such as black smoke is reproduced through ignition of the collected particulates at an end thereof and subsequent propagative combustion and incineration of the particulates. Thus, the reproduction of the filter is effected incompletely. Thereafter, particulates are collected using the reproduced filter in such a state that the residual particulates are abnormally deposited on the filter. When the filter is subsequently reproduced again or when the particulates deposited on the filter are self-ingited, the filter is consequently overheated to a temperature of at least 1,000.degree. C. to cause fusion damage to the filter, cracking of the filter, etc., resulting in breakage of the filter.
An inorganic fiber is used to produce a conventional diesel particulate filter. The use of the inorganic fiber instead of a monolithic porous material as the material of the filter can prevent the filter from undergoing damage such as cracking due to thermal stress.
Where a filter is to be made of an inorganic fiber in view of the foregoing, the inorganic fiber is required to have properties such as a heat resistance, oxidation resistance, NO.sub.x resistance, SO.sub.x resistance, flexibility and fiber fabricability so as to permit the fibers to be formed into a filter. A silicon carbide ceramic is mostly used as the material of the inorganic fiber capable of exhibiting such properties.
However, a filter body made using an inorganic fiber of such a silicon carbide ceramic, though advantageous in the foregoing respect, involves a problem of high cost because polytitanocarbosilane or polycarbosilane is used as the starting material of the inorganic fiber constituting the filter body. There is further a fiber comprising alumina and silica as inexpensive materials of inorganic fibers, which is however problematic in respect of properties such as heat resistance and flexibility.
Meanwhile, the applicant of the instant application developed a diesel particulate filter made of an inorganic continuous filament, and previously filed a patent application directed thereto which is Japanese Patent Application No. 114,131/1994. This diesel particulate filter is produced by horizontally irregularly orienting and laminating cut pieces of a heat-resistant inorganic continuous filament cut to a predetermined length, needling the resulting laminate to effect vertical entanglement of the inorganic continuous filament, superposing wire nets made of a heat-resistant metal on both the upper and lower surfaces of the resulting felting material, and fastening them from both surfaces thereof with a heat-resistant yarn. Herein, the above-mentioned inorganic continuous filament is a fiber made of a material selected from among Si-Ti-C-O, Si-C-O, Si-N, SiO and metals, and having the surfaces thereof coated with at least one of silicon carbide, aluminum and alumina.